If these are the type of shots you regularly want to get, some sort of stabilization seems critical (and this is coming from someone who never uses a tripod either.)

I mean you could decide to get a full-frame camera, but the NEX-5N is not really the weak link in the chain here- it's our hands. You could get over a stop improvement by going full-frame by spending quite a bit of money, or you could get a tripod, gorillapod, monopod for $30-$50 and get 3-4-5-6 stops improvement.

Even with full frame you are going to run into limits hand-held. But if you have some physical support your range of possiiblity expands dramatically. Maybe you even do HDR for a couple of those shots, where you want to catch the outline of the buildings in the distance more clearly, but don't want to blow out the lit windows nor the lit streets in the foreground.

And definitely try the handheld twilight mode. I don't use it a lot myself, but I think it was pretty much designed for just these kinds of situations.